1. Technical Field
The present invention relates to an arrangement for stabilizing the gain-bandwidth product (gm/C) of analog circuits including bipolar devices in which bipolar devices define the gm and, more particularly, to a stabilization arrangement capable of essentially eliminating variations in gm/C related to processing and temperature variations.
2. Description of the Prior Art
A problem arises in the design of certain analog circuits including bipolar devices in which bipolar devices define the gm, such as operational amplifiers (op-amps), continuous time filters, buffers, gain stages and the like. Preferably, the signal bandwidth, stability (Q and phase margin), and noise bandwidth for such devices should be well-controlled for a specified range of operating conditions. However, the gain-bandwidth product (which is equivalent to the ratio of gm/C, as seen below) for these devices, as well as for other analog circuits conducting DC current, is known to change with variations in both processing parameters and ambient temperature. In particular, the gain-bandwidth product for an op-amp is defined as the product of the amplifier's DC gain and the frequency of its dominant pole. For a typical single stage op-amp, for example, the DC gain is defined as gm/go (gm being the input transconductance and go being the output conductance) and the pole is defined as occurring at the frequency go/C (C being the amplifier capacitance at the dominant pole node). Therefore, multiplying (gm/go) by (go/C) yields a "gain-bandwidth product" of gm/C. If the gm/C value for a given analog circuit with a gm defined by bipolar devices can be kept stable over a given range of processing parameters and temperature, the circuit becomes better controlled. For more complicated circuits, the gain bandwidth product can often be reduced to a gm/C type of ratio. Thus, any arrangement that provides additional stability to the gain bandwidth product is useful for a variety of applications.
There are techniques in the prior art that address the desire to stabilize the transconductance or the gain-bandwidth product. For example, it is known to bias bipolar operational amplifiers with currents that are proportional to absolute temperature (PTAT), to counter a reduction in gm with respect to an increase of temperature. Control of temperature-dependent variables is also described in an article entitled "A 27 MHz Programmable bipolar 0.05.degree. Equiripple Linear-Phase Lowpass Filter", by G. A. Veirman et al. appearing in the Proceedings of the 1992 ISSCC at page 64 et seq. Additionally, U.S. Pat. No. 4,484,089 issued to T. R. Viswanathan on Nov. 20, 1984 discloses a switched-capacitor arrangement for conductance and transconductance control of variable elements. Techniques for addressing transconductance in MOS circuits is disclosed in U.S. patent application Ser. No. 08/741,067 entitled "Method of Biasing MOSFET Amplifiers for Constant Transconductance", filed Oct. 30, 1996, and U.S. patent application Ser. No. 08/792,823 entitled "Stabilization of Gain-Bandwidth Product in Analog Circuit Devices", filed Jan. 30, 1997, describes particular arrangements suitable for stabilizing the gain-bandwidth product of MOS circuits, where both of these applications are assigned to the assignee of record for the present application.
There remains in the prior art, however, a need for an arrangement capable of stabilizing the gain-bandwidth product of analog circuits whose gm is defined by bipolar devices.